Earlier, The Sunday Times reported that 12 senior staff working for Jeremy Corbyn and shadow chancellor John McDonnell were members of social media groups containing anti-Semitic and violent comments.

The paper said an investigation into 20 of the biggest pro-Corbyn Facebook groups – numbering around 400,000 members – had uncovered routine attacks on Jewish people, including Holocaust denial.

Working with whistleblowers who were able to gain access to restricted membership groups, it said that it had uncovered more than 2,000 racist, anti-Semitic, misogynistic, violent and abusive messages.

A Labour source said such sites routinely received hundreds of postings a day, most of which were perfectly innocent messages about party policies or events.

Many of the staff concerned were either no longer active on Facebook or were unaware they were members of these and had not seen the content highlighted by the paper.

Phil Noble / Reuters

Following the investigation, one of Labour’s biggest private donors launched a bitter attack on the party’s failure to deal with anti-Semitism under Corbyn.

Sir David Garrard, who has donated around £1.5 million since 2003, said he had now left the party having seen it fail to respond to “the most blatant acts of anti-Semitism”.

Last week, Corbyn apologised for arguing on Facebook in 2012 against the removal of an anti-Semitic mural that depicted a group of Jewish men playing Monopoly on the backs of a group of emaciated people.

Corbyn admitted “regret” after he made a “general comment” that appeared to defend the mural, which drew criticism from his own MPs.

The Labour leader has pledged to root out anti-Semitism within Labour, but has faced a troubled week that started with Jewish leaders demonstrating outside parliament.